PerspectivePlant Biology

Hormones and the Green Revolution

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Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 71-72
DOI: 10.1126/science.1090811

In his famous 1840 manifesto, Justus Liebig [HN1] advocated a more rational approach to agronomic research. He proposed integrating physiological and chemical principles to improve the yield of crop plants. However, it was not until after World War II that agronomists, searching for better herbicides and insecticides (1), took a rational approach to boosting crop yields. The creation of genetically improved crop varieties, in particular dwarf cultivars of wheat and rice (see the figure), resulted in the doubling of food grain production within a few decades, a success dubbed the green revolution (2) [HN2]. The short stature and sturdy stalks of the dwarf varieties of wheat and rice [HN3] rendered them resistant to flattening by wind, rain, or high densities (lodging) and more effective in converting fertilizer input into higher yields. For example, short semi-dwarf wheat varieties exhibit a 100% greater yield than earlier, taller cultivars (3). The genes responsible for the rice and wheat green revolution dwarf varieties have been identified. They encode proteins that either regulate synthesis of the plant growth hormone [HN4] gibberellin or modulate its signaling pathway [HN5]. On page 81 of this issue, Multani et al. (4) [HN6] identify a new mechanism controlling plant height in maize [HN7] and sorghum dwarf [HN8] mutants of agronomic importance. They show that the mutant gene in the brachytic2 [HN9] (br2) maize mutant and the dwarf3 (dw3) sorghum mutant encodes a protein responsible for the transport of auxin [HN10], the first plant growth hormone to be discovered.

The benefits of being short.

(A) Varieties of wheat from the early 1900s were almost as tall as a normal person. (B) In contrast, modern wheat varieties, particularly those that launched the green revolution, have stalks that are 40 to 100% shorter than the stalks of earlier cultivars. The short wheat varieties carry mutations in genes encoding proteins that regulate the synthesis and signaling of gibberellin (GA), a plant growth hormone. (C) The pathway of GA synthesis indicating mutants of agronomic interest in which GA synthesis or signaling is disrupted. GA directs degradation of the product of the GAI (gibberellic acid insensitive) gene, which is a transcriptional repressor. In gai mutants (rht, reduced height; slr, slender; sd, semidwarf), degradation of GAI is abrogated and GA-inducible genes are repressed (9). (D) Gene mutations affecting transport of auxin, some of which contribute to dwarfism. (PIN, pinoid phenotype; ABC, ATP-binding cassette; MDR, multidrug resistance; PGPs, P-glycoproteins; br, brachytic; dw, dwarf).

A rational approach to improving crop varieties depends on a better understanding of plant biology. Of the tens of thousands of genes in plant genomes, one needs to know not only which genes control complex traits like size or yield but also how these genes limit successful plant-breeding programs. There are several strategies for elucidating how genes affect the phenotypes of plants. One approach is to analyze quantitative trait loci (QTL) [HN11], which reveal allelic variations that can be exploited in plant breeding (5). Another approach is to screen plant genomes for a genetic signature, which indicates strong selection for a specific set of genes that generates modified and agronomically beneficial alleles. Such an approach uses statistical methods to reveal significant nucleotide diversity between wild plants and their domesticated counterparts (6) [HN12], enabling researchers to identify genes of agronomic importance. A third approach, and the one exploited by Multani and colleagues, is to analyze plant mutants that exhibit improved growth characteristics and then to identify the responsible gene mutation.

In their study, Multani et al. (4) reveal how the growth hormone auxin controls plant height (see the figure). Auxin regulates a plethora of growth and developmental processes in plants, and its effects depend partly on its asymmetric transportation from the shoot apex along the shoot (basipetal [HN13] transport). Putative protein transporters of auxin include efflux carriers encoded by the pinoid phenotype (PIN) gene family (7). When the model plant Arabidopsis [HN14] is treated with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA), multiple morphological changes result and the treated plant resembles the PIN mutant phenotype. In Arabidopsis mutants that exhibit defective auxin transport due to disrupted accumulation of PIN proteins at the basal ends of cells (8), the genetic mutations have been identified. Mutations have been found in two genes encoding proteins that are homologous to the P-glycoprotein type of ABC transporters [HN15]. Multani and colleagues now reveal that the br2 and dw3 mutant varieties carry mutations in genes that are homologous to the P-glycoprotein genes of Arabidopsis. The basipetal transport of auxin is reduced in br2 maize and dw3 sorghum dwarf mutants exposed to light. However, in contrast to its broad morphological effects in Arabidopsis, defective auxin transport in the br2 and dw3 mutants specifically results in shortening of lower stalk internodes and dwarfism.

The dw3 mutant has been integrated in sorghum plant-breeding programs for more than 50 years. Now that the mechanism of dwarfing in the br2 mutant has been identified, the br2 gene can be manipulated to reduce the height of maize cultivars. The discovery of genes that increase the yield of crop plants, coupled with genetic engineering, should enable the production of even better crop varieties in the near future.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Web Collections, References, and Resource Lists

Academic Info provides links to Internet resources on botany and plant biology.

The Google Directory offers links to Internet resources in plant molecular biology and plant physiology.

The Plant Link Library is provided by the Department of Plant Sciences, Wageningen University, Netherlands.

The American Society of Plant Biologists provides a collection of Internet resource links.

The Plant Science HomePage of the Agriculture Network Information Center (AgNIC) provides links to Internet plant science resources. The University of Maryland's AgNIC gateway is a guide to agricultural biotechnology information on the Internet.

The Plant Genome Database (PlantGDB) is provided by the Brendel Research Group, Department of Genetics, Development and Cell Biology, Iowa State University.

Gramene is a data resource for comparative genome analysis in the grasses.

Online Texts and Lecture Notes

A presentation on plant transformation is provided by the Partnership for Plant Genomics Education, University of California, Davis.

Botany online is an Internet hypertextbook by P. von Sengbusch, Faculty of Biology, University of Hamburg, who also maintains the Internet Library, a resource for teaching botany and related topics.

A Plant Physiology Information Site is provided by R. Konig, Biology Department, Eastern Connecticut State University.

R. Michelmore and A. Fischer, Department of Vegetable Crops, University of California, Davis, make available lecture notes for a course on plants and people. Included are A. Fischer's lecture notes on the Green Revolution.

A. Caines, Department of Cell Biology and Molecular Genetics, University of Maryland, makes available lecture notes for a plant biology course.

P. Hayes, Department of Crop and Soil Science, Oregon State University, makes available lecture notes for a course on plant genetics.

P. McClean, Department of Plant Sciences, North Dakota State University, provides lecture notes for a course on plant molecular genetics.

B. Fristensky, Department of Plant Science, University of Manitoba, Canada, provides lecture notes for a course on plant molecular genetics.

J. Haseloff, Department of Plant Sciences, University of Cambridge, provides lecture notes and other resources for courses in plant development and biotechnology.

E. Wurtele, Department of Ecology, Evolution and Organismal Biology, and P. Becraft, Agronomy Department, Iowa State University, provide lecture notes for a course on plant growth and development.

General Reports and Articles

The American Society of Plant Biologists provides free access to selected articles in The Plant Cell and Plant Physiology.

Cherrybyte.org makes available an article by B. Clarke titled “Plants become downsized by a dwarfing gene.”

The 2 May 2003 issue of Science had a review by R. E. Evenson and D. Gollin titled “Assessing the impact of the Green Revolution, 1960 to 2000.”

The 16 July 1999 issue of Science (a special issue on plant biotechnology) had a review by C. Somerville and S. Somerville titled “Plant functional genomics” and a review by B. Mazur, E. Krebbers, and S. Tingey titled “Gene discovery and product development for grain quality traits.”

The 20 December 2002 issue of Science had a review by A. M. Glazier, J. H. Nadeau, and T. J. Aitman titled “Finding genes that underlie complex traits.”

Numbered Hypernotes

1. An entry on Justus Liebig is included in the Columbia Encyclopedia and the Britannica Concise Encyclopedia. The Liebig-Museum in Giessen offers a profile of Liebig in English and more detailed information in German. The Rare Book Room Exhibit at the University of Illinois provides a presentation on Liebig's Die Organische Chemie in ihre Anwendung auf Agricultur und Physiologie (Braunschweig, 1840). P. von Sengbusch's Botany online discusses Liebig's work on plant physiology. Liebig's Chemical Letters (Letters XII and following relate to agriculture) are made available by P. Childs, Department of Chemical and Environmental Science, University of Limerick, Ireland. The NewCROP Web site of the Center for New Crops and Plant Products at Purdue University makes available lecture notes for a course on the history of horticulture; information about Liebig's influence is included in the reading on early roots of the organic movement.

2. The Green Revolution. An entry on the Green Revolution is included in the Columbia Encyclopedia and in the Britannica Concise Encyclopedia. Nature Reviews Genetics makes available a slide presentation and links from the October 2001 review article by G. S. Khush titled “Green revolution: The way forward” (2). A presentation on the Green Revolution is provided by the Environmental Literacy Council. The Agricultural Research Service offers a presentation on the Green Revolution. The 1970 Nobel Peace Prize was awarded to Norman Borlaug, a central figure in the Green Revolution; his Nobel lecture was titled “The Green Revolution, peace, and humanity”; his 2000 anniversary lecture titled “The Green Revolution revisited and the road ahead” is available in PDF format. The 11 February 1983 issue of Science had an article by N. E. Borlaug titled “Contributions of conventional plant breeding to food production” (3). R. Salvador, Agronomy Department, Iowa State University, offers lecture notes on the Green Revolution for a course on world food issues. The FAO Newsroom makes available a March 2003 Focus on the Issues article titled “Crop breeding: The Green Revolution and the preceding millennia.” The International Food Policy Research Institute makes available in PDF format a 2002 issue brief titled “Green Revolution: Curse or blessing?”

3. Presentations on wheat and rice are offered by the International Starch Institute. NewCROP provides information on wheat and rice. Plant Genome Central at the National Center for Biotechnology Information (NCBI) offers information on rice and wheat projects. J. McKenna, Crop and Soil Environmental Sciences Supplemental Learning Center, Virginia Polytechnic Institute and State University, offers lecture notes on wheat and rice for a course on world crops and cropping systems. The U.S. Wheat Genome Project Web page offers a project description, as well as education and general Internet resources. Rice Web is an information resource from the International Rice Research Institute. Oryzabase is a comprehensive rice science database. GrainGenes is a compilation of molecular and phenotypic information on wheat, barley, rye, triticale, and oats.

4. Plant hormones. M. Farabee's On-Line Biology Book includes a chapter on plant hormones, nutrition, and transport. The Department of Horticulture and Crop Science, Ohio State University, provides an introduction to plant hormones in the online resource page for a plant biology course. G. Anderson, Department of Biological Sciences, University of Southern Mississippi, offers a presentation on plant hormones for a biology course. P. von Sengbusch's Botany online includes a chapter on plant hormones and growth regulators. D. Cipollini, Department of Biological Sciences, Wright State University, offers lecture notes on plant hormones for plant physiology course. B. Fristensky provides lecture notes on hormonal regulation of gene expression and development for a course on plant molecular genetics.

5. Gibberellin and its signaling pathway. An entry on gibberellin is included in the Columbia Encyclopedia and in the Hutchinson Encyclopedia. Kimball's Biology Pages offer an introduction to gibberellins. P. von Sengbusch's Botany online includes a section on gibberellins. The Plant-hormones.info Web site offers a presentation on gibberellin. E. Iglich, Biology Department, Western Maryland College, offers lecture notes on gibberellin for a botany course. The Department of Plant Physiology and Biochemistry, Institute of Plant Biology, Technical University of Braunschweig, Germany, offers a research presentation on gibberellin and its signaling pathway. The May 2002 supplement (on plant hormones and signaling) of The Plant Cell had a review article by N. Olszewski, T. Sun, and F. Gubler titled “Gibberellin signaling: Biosynthesis, catabolism, and response pathways.”

6. D. S. Multani and M. Chamberlin are at Pioneer Hi-Bred International, Inc., Johnston, IA. S. P. Briggs is at Diversa Corporation, San Diego. J. J. Blakeslee and A. S. Murphy are in the Department of Horticulture, Purdue University. G. S. Johal is in the Department of Botany and Plant Pathology, Purdue University.

7. Maize. An introduction to maize is provided by the International Starch Institute. The Maize Genome Sequencing Projects Web site provides an introduction to the maize plant. NewCROP provides information about maize. The Maize Genetics and Genomics Database (MaizeGGD), the successor to the Maize Database (MaizeDB), is a central repository on the genetics and molecular biology of maize; links to related Internet resources and educational resources are provided. Crop Plant Resources from the Molecular Biology Program, New Mexico State University includes a presentation on maize. J. McKenna makes available lecture notes on corn (maize) for a course on world crops and cropping systems. The Maize Page is provided by the Agronomy Department, Iowa State University.

8. Sorghum. The USDA's Plants Database provides information on sorghum and its species. The International Starch Institute provides an introduction to sorghum. Crop Plant Resources includes a presentation on sorghum. NewCROP provides information resources on sorghum. J. Vorst, Department of Agronomy, Purdue University, makes available information on grain sorghum for an agronomy course on crop production. J. McKenna makes available lecture notes on sorghum for a course on world crops and cropping systems. The NCBI Taxonomy Browser provides links to sorghum resources.

9. Entries for brachytic2 and for brachytic plant (phenotype) are included in MaizeGDB.

10. Auxin and auxin transport. Auxin is defined in the online Hutchinson Encyclopedia and in the Columbia Encyclopedia. Kimball's Biology Pages offer a presentation on auxin. P. von Sengbusch's Botany online includes a section on auxins. The Plant-hormones.info Web site offers a presentation on auxin. A. S. Murphy offers a research presentation on auxin. E. Iglich offers on lecture notes on auxin for a botany course. The Iowa State University course on plant growth and development makes available lecture notes on auxin transport. The 18 December 1998 issue of Science had a Perspective by A. M. Jones titled “Auxin transport: Down and out and up again.” The February 2002 issue of The Plant Cell had an article by G. K. Muday and A. S. Murphy titled “An emerging model of auxin transport regulation.”

11. Quantitative trait loci. A definition of quantitative trait loci (QTL) is provided in B. Schlindwein's Hypermedia Glossary of Genetic Terms. P. McClean provides lecture notes on QTL for a course on plant molecular genetics. B. Fristensky offers lecture notes on QTL for a course on plant molecular genetics. M. Whitlock, Department of Zoology, University of British Columbia, makes available lecture notes on QTL in the quantitative genetics section of a course on population and quantitative genetics.

12. Screening approach. The 23 July 2002 issue of the Proceedings of the National Academy of Sciences had an article by Y. Vigouroux, J. Doebley et al. titled “Identifying genes of agronomic importance in maize by screening microsatellites for evidence of selection during domestication” (6).

13. Basipetal is defined in the GardenWeb glossary.

14. The model plant Arabidopsis. The Arabidopsis Information Resource (TAIR) provides an introduction to Arabidopsis thaliana. J. Haseloff offers information about Arabidopsis as a model plant. Genome Biology makes available a 31 July 2001 meeting report by B. Kost titled “Towards a virtual Arabidopsis plant.” The American Society of Plant Biologists' Arabidopsis Book is available on BioOne. The January 2001 issue of Plant Physiology had an article by E. Meyerowitz titled “Prehistory and history of Arabidopsis research.”

15. P-glycoprotein type of ABC transporter. P-glycoprotein (PGP) is defined in the On-line Medical Dictionary and in the Medical Dictionary Online. A brief introduction to PGP is provided by M. Blaxter, University of Edinburgh, UK, for a genetics course. An introduction to PGP is provided by the Mental Health Connections, Inc. Web site. Kimball's Biology Pages offer an introduction to ABC transporters. The Transport Protein Database provides a presentation on the ABC superfamily. TAIR offers a list of PGP and other ABC transporters in Arabidopsis. The November 2001 issue of The Plant Cell had an article by B. Noh, A. S. Murphy, and E. P. Spalding titled “Multidrug resistance-like genes of Arabidopsis required for auxin transport and auxin-mediated development.” Purdue University issued a 25 June 2003 press release titled “Purdue genetic discovery may aid plants and human medicine” about research on auxin and PIN1 in Arabidopsis published in Nature (8).

16. Francesco Salamini is in the Department of Plant Breeding and Yield Physiology, Max Planck Institute for Plant Breeding Research, Köln, Germany.

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